vb.net 实现多态

Public Module Module2
    Public Interface MyInterface
        Property stuName As String
        Function GetScore(ByVal x As Single) As Single
    End Interface

    Public Class StuInfo
        Implements MyInterface
        Private studentScore As Single
        Private studentName As String
        Public Property Score() As Single
            Get
                Return studentScore
            End Get
            Set(ByVal value As Single)
                studentScore = value
            End Set
        End Property
        Public Function GetScore(ByVal x As Single) As Single Implements MyInterface.GetScore
            Return x * 0.8
        End Function

        Public Property StuName() As String Implements MyInterface.stuName
            Get
                Return studentName
            End Get
            Set(ByVal value As String)
                studentName = value
            End Set
        End Property
    End Class


    Public Class StuMessage
        Implements MyInterface
        Private studentName As String
        Private stuScore As Single
        Public Property Score() As Single
            Get
                Return stuScore
            End Get
            Set(ByVal value As Single)
                stuScore = value
            End Set
        End Property
        Public Function GetScore(ByVal x As Single) As Single Implements MyInterface.GetScore
            Return x * 0.8
        End Function

        Public Property StuName() As String Implements MyInterface.stuName
            Get
                Return studentName
            End Get
            Set(ByVal value As String)
                studentName = value
            End Set
        End Property

    End Class

End Module

Public Class TestClassForm
    Public Sub ShowScore(ByVal obj As MyInterface, ByVal s As String, ByVal score As Single)
        MsgBox(s & "成绩为:" & obj.GetScore(score) & "")
    End Sub
    Private Sub TestClassForm_Load(sender As Object, e As EventArgs) Handles MyBase.Load

    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim obj1 As New StuInfo()
        Dim obj2 As New StuMessage()
        ShowScore(obj1, "期中", 100)
        ShowScore(obj2, "期末", 100)
    End Sub
End Class